Various devices are known for measuring displacement or strain. For instance, in materials testing, devices known as extensometers are frequently used to measure strain (defined as a change in length (ΔL) over original length (L); or ΔL/L) of a test specimen. Although different configurations are known, the term “extensometer” may refer to most any device that incorporates a sensor capable of measuring relatively small increments of deformation of the test specimen.
One type of extensometer is a mechanical or “contact” extensometer. These devices use spaced-apart arms having knife edges or other contact elements at their distal ends. The arms may be joined to one another at or near their proximal ends via a flexible transducer assembly. The knife edges are attached to the test specimen at a preset distance from one another (the “gauge length” of the extensometer). During testing, the knife edges move with the test specimen as the latter deforms. The deformation or strain is then detected and measured, typically with strain gauges contained within the transducer assembly.
While effective, contact extensometers are not well-suited for all testing applications. For instance, extremely fragile materials may present problems for both extensometer attachment and measurement. Moreover, contact extensometers—when used with materials that fail violently—may yield inaccurate readings at or near specimen failure, and, moreover, may be damaged by the failing specimen. Still further, contact extensometers require physical attachment to, and detachment from, each test specimen. Such a repetitive procedure may slow throughput when multiple test specimens are sequentially tested.
To address these and other concerns, optical or “non-contact” extensometers are sometimes used. These extensometers may utilize cameras to optically detect deformation by, for example, monitoring marks on the specimen from a remote, non-contacting location.
While effective, non-contact extensometers are prone to various issues. For instance, with non-contact extensometers, it is often difficult to set a desired working distance from the extensometer sensor to the test specimen in an accurate and repeatable manner. As a result, tedious and time-consuming setup and adjustment may be required with each test specimen.